Federal, state and local environmental laws require significant reduction of discharge of harmful volatile organic compounds (VOCs) into the atmosphere. Many of these VOCs are known, or suspected, to cause cancer or other serious health effects. They are also often harmful to the environment. Non-limiting examples of VOCs include benzene, which is found in gasoline; perchlorethylene, which is emitted from some dry cleaning facilities; and methylene chloride, which is used as a solvent and paint stripper by a number of industries.
Most of these VOCs come from human-made sources, including mobile sources (e.g. cars, trucks, buses), stationary sources (e.g. factories, refineries, power plants, etc.), as well as indoor sources (e.g. some building materials and cleaning solvents). Great strides have been made in reducing the level of VOCs from industrial and commercial sources, particularly from large stationary sources such as chemical plants, oil refineries, aerospace manufacturers, steel mills, and paper mills. Great strides have also been made in reducing the level of benzene and toluene that are released with exhaust from automobiles and trucks. Although substantial progress has been made to reduce the level of VOCs from high volume industrial and commercial sources, there still remains a need to reduce the release of VOCs from the myriad lower volume sources, such as tanks used to transfer and/or store hydrocarbon liquids. One example of such a tank is the so-called “frac” tank that is typically used for on-site temporary storage of liquid hydrocarbons. Another example of such a tank is the tank mounted on a so-called “vacuum truck” that has various uses, such as for chemical cleanup and the transfer and transport of hydrocarbon liquids. During the transfer of hydrocarbon liquids into and out of such tanks, some of the hydrocarbon will vaporize, and if not prevented, will enter the atmosphere with air that is vented, such as when the tank is being filled with a liquid organic material. While a single tank would not represent a large release of VOCs, the cumulative effect of many such tanks would represent a substantial release of VOCs unless such VOCs were extracted from the air before being vented into the atmosphere.
VOCs are conventionally captured from such sources by passing the vented gaseous stream through one or more canisters containing a bed of sorbent material, such as activated carbon. The problem with such as technique is that, because of the substantial amount of VOCs in the vent gas, the carbon material quickly becomes spent, thus requiring frequent change-out and/or regeneration of the carbon bed.
Therefore, there remains a need in the art for equipment and methods for removing VOCs from gases, typically air, that are being vented into the atmosphere from various holding tanks, both stationary and mobile.